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POUCH   OF   MARSUPIALS

secondary. If this fact be fairly to be interpreted in the sense
which Dr. Klaatseh attaches to it, we have an interesting case
of the growth of a new organ out of and partly replacing an
old organ. In the Monotremes there is a pouch which facilitates
or performs both nutritive and protective functions; in the
Phalanger these two functions are carried on in separate
pouches ; finally, in other Marsupials, there is a return to the
undifferentiated state of affairs found in the Monotremata, but
with the help of a new
organ not found in them.

Though so character-
istic of Marsupials, the
marsupial pouch is not
always developed in them.
It is present in all the
Kangaroos, Wallabies, and
Wombats, in fact in the
Diprotodonts. It is also
present in a number of
the carnivorous Polypro-
todont Marsupials ; but in
Pliascologale it is only pre-
sent in rudiment, and in
Myrmecobius it is entirely
obsolete. In the American
Opossums the state of the
pouch is variable. " Gener-
ally absent, sometimes
merely composed of two
lateral folds of skin separ-

. 4.óDiagram of the development of the nipple
(in vertical section). A, Indifferent stage, gland-
ular area flat; B, elevation of the glandular area
with the nipple ; C, elevation of the periphery
of the glandular area into the false teat, a,
Periphery of the glandular area; 5, glandular
area ; gl-> glands. (From Gegenbaur.)

ate at each end, rarely complete," is Mr. Thomas* summary in his
definition of the family Didelphyidae.1 Another curious feature
of the pouch in the Marsupials is the variability in the position
of the mouth of the pouch: in all the Diprotodonts it looks
forward; but in many Polyprotodonts it looks backward. This,
however, has some connexion with the habitual attitude of the
possessor: in the Kangaroo, leaping along on its hind-legs, it is
requisite that the pouch should open forwards; but in the
dog-like Thylacine, going on all fours, the fact that the pouch
1 Catalogue of Marsupials in British Museum, 1886.
VOL. X                                                                                                       C